Freshwater ecosystem structure and function are tightly linked to the watershed or
catchment of which they are a part. Because riverine networks, lakes, wetlands, and their
connecting groundwaters, are literally the ‘‘sinks’’ into which landscapes drain, they are
greatly influenced by terrestrial processes, including many human uses or modifications of
land and water. Freshwater ecosystems, whether lakes, wetlands, or rivers, have specific
requirements in terms of quantity, quality, and seasonality of their water supplies. Sustainability
normally requires these systems to fluctuate within a natural range of variation.
Flow regime, sediment and organic matter inputs, thermal and light characteristics, chemical
and nutrient characteristics, and biotic assemblages are fundamental defining attributes of
freshwater ecosystems. These attributes impart relatively unique characteristics of productivity
and biodiversity to each ecosystem. The natural range of variation in each of these
attributes is critical to maintaining the integrity and dynamic potential of aquatic ecosys